The present invention relates to an electromagnetic flowmeter for detecting the flow rate of a conductive fluid in a process control apparatus.
A detection signal corresponding to the flow rate of a conductive fluid flowing in a magnetic field can be obtained by sampling voltages generated from electrodes in contact with the fluid at a predetermined timing. This principle is used for an electromagnetic flowmeter.
Since electrochemical noise is superposed on the detection signal, the polarities of an excitation current are alternately changed to remove the noise. However, if the polarities are alternately changed, it takes time to stabilize the state of the excitation current. For this reason, whenever the excitation polarity is changed, sampling is performed after a stabilization time at which the state is stabilized, thereby obtaining a detection signal.
Such an operation requires an excitation current switching timing at which the polarities of an excitation current are switched, and a sampling timing at which sampling is performed. Both the timings are generated by a timing generating means incorporated in the flowmeter.
However, in addition to the electrochemical noise described above, external noise (since most of the external noise is commercial power supply noise, the noise will be referred to as commercial power supply noise hereinafter) may also be superposed on the detection signal. If this noise is small, no problem is posed. If, however, the amplitude of the commercial power supply noise is large, since the noise is not synchronized with internal timing signals, conspicuous variations or fluctuations in output are caused. In some case, periodic variations with a long duration called beats are caused, interfering with measurement.
If a flowmeter is designed to be operated by a commercial power supply, a timing signal and commercial power supply noise are synchronized with each other by forming the timing signal from the commercial power supply. Therefore, a noise reduction effect can be expected. However, commercial power supply noise is often mixed with noise from another power supply system, the noise reduction effect is not as high as expected in many cases.
In addition, if the flowmeter is designed to be operated by a DC power supply, since there is no signal synchronized with a commercial power supply, variations in output tent to occur.
Since a timing signal is generated on the basis of an external noise signal detected by the noise detecting means, the timing signal and the external noise signal are synchronized with each other. Therefore, a noise reduction effect can be obtained. Even if no external noise signal can be detected during a given period for some reason, a pseudo-noise signal is continuously generated by the pseudo-noise generating means during the period. A timing signal is generated on the basis of this pseudo-noise signal. When the period during which the pseudo-noise signal is generated is ended, a timing signal is generated on the basis of the external noise signal detected by the external noise detecting means.